Many products, for example food, electronic devices and pharmaceuticals, are very sensitive to moisture and oxidizing agents. Many of these products rapidly degrade when exposed to water, oxidizing agents or other gases or liquids. Polymeric substrates, such as polymeric foils, are often used to package these products. These foils frequently exhibit a permeability for water vapor and for oxidizing agents in the range of more than 1 g/(m2*day). This high degree of permeability is unacceptable for most of the products packaged in polymer foils.
One packaging application that uses polymeric substances is the packaging of organic electroluminescent devices (OLEDs). An OLED device includes a functional stack formed on a substrate. The functional stack includes at least one organic functional layer sandwiched between two conductive layers. The conductive layers serve as electrodes (cathode and anode). When a voltage is applied to the electrodes, charge carriers are injected through these electrodes into the functional layers and upon recombination of the charge carriers, visible radiation can be emitted (electroluminescence). The functional stack of the OLED is very sensitive to moisture and oxidizing agents, which can cause oxidation of the metals of the electrodes or deterioration of the organic functional layers. The next generation of organic electroluminescent devices are likely to be arranged on flexible substrates, such as polymeric substrates, and are under current investigation. For a sufficient OLED lifetime, polymeric substrates with a permeability for water or oxidizing agents below 10−6 g/(m2 day) are desirable.
Patent application WO 00/48749 A1 describes a method of reinforcing polymeric foils with thin ceramic barrier layers in order to block out gases or liquids more efficiently than when only polymeric foils are used. Ceramic layers frequently have defects in their microstructures that can serve as continuous paths for gases and water vapor to pass through the ceramic barrier layers. These defects lead to a decreased ability of the ceramic barrier layers to serve as a barrier. In this context, all pathways through the inorganic, ceramic barrier layers are called defects. Defects in the context of this specification include pinholes, grain boundaries, shadowing effects, and impurities, as well as other imperfections in a material.
Patent publication WO 01/81649 A1 describes a method of depositing several thin ceramic barrier layers on top of each other on polymeric substrates to enhance the barrier abilities of the polymeric substrates. This publication suggests decoupling defects in successive ceramic barrier layers by changing the deposition parameters and growth conditions for the deposition of the ceramic barrier layers. According to the publication, this method should lead to mismatched subsequent barrier layers that exhibit different microstructures and therefore the paths for gases and water vapor permeation are degraded, leading to enhanced barrier abilities. Experiments carried out by the inventors indicate that ceramic barrier stacks produced by this method exhibit enhanced barrier abilities, but show no major improvements over applying a single ceramic barrier layer when defects larger than grain boundaries, e.g., pinholes or shadowing effects, are present.